The present invention relates to a new and improved method of, and apparatus for, the mechanical-thermal dewatering of a water-containing fibrous or fiber web, especially although not exclusively, a paper web.
Generally speaking, in the practice of the invention for the dewatering of the fibrous web there is simultaneously utilized pressure and thermal energy in a nip through which there is guided the fibrous web to be dewatered in conjunction with a suitable porous band, such as typically a felt, for the absorption of the expressed water released from the fibrous web. The application of thermal energy or heat advantageously increases the quantity of water which can be displaced from the fibrous or fiber web and taken up or absorbed at the porous band.
Such dewatering operations are utilized, by way of example, in paper making machines at the press or pressing section or at a region subsequent thereto viewed with respect to the direction of travel of the fibrous web. There are already known in such technology different constructions of the press or pressing section wherein there is employed thermal energy at the region of the nip.
In U.S. Pat. No. 3,354,035, granted Nov. 21, 1967, there is disclosed for instance the possibility of bringing a moist paper web into contact with a heated cylinder. The paper web is maintained in contact with the cylinder surface at temperatures up to 200.degree. C. by means of a pervious pressing band. In such document there is mentioned the advantage that the water within the paper web can be entrained into the felt by the formed water vapor or steam. This water vapor or steam augments the transfer of the water out of the paper web. There is mentioned therein the danger of damage to the paper web by virtue of the press rolls which are considerably heated.
On the other hand, when utilizing bands or belts as the pressing or contact elements, by virtue of the low-contact pressure the augmenting thermal action is not completely effective, so that the dewatering of the paper web is not particularly efficacious.
In U.S. Pat. No. 4,324,613, granted Apr. 13, 1982, it is proposed to obtain the requisite extremely high thermal energy density or flow rates by externally heating a cylinder by means of a gas burner. During arrival of the paper web in a nip formed with a suction roll there is produced water vapor or stream which displaces the free surface water out of the paper web into a felt which is then withdrawn over the suction roll. The paper web is trained about the heated cylinder and is scraped away therefrom. The preferred utilization of this method resides in fabrication of paper of very low weight.
In PCT-International Published Application No. WO 85/03314, published Aug. 1, 1985, a moist paper web is pressed by means of a pressing felt heated to a temperature exceeding 100.degree. C. against a drying cylinder. The water which has been pressed into the felt is subsequently sucked out of the felt in conjunction with the condensed water vapor or steam. The produced water vapor in the pressing nip is supposed to simultaneously constitute a barrier against the re-wetting of the paper web. The required heating of the felt by means of a burner appears to be not without considerable danger and, as expected, the water vapor or steam exploding behind the press nip can cause damage of the outbound or outgoing paper web.
In the Tappi Journal, Volume 66, No. 9, September 1983 issue, pages 123 to 126, there is described a dewatering press working with a heated roll press nip. Experiments with a press simulator demonstrated that with a heated press surface there can be attained an appreciable increase in the drying action in contrast to pure mechanical dewatering. This action, referred to in the art as "Impulse Drying" is explained in terms of the water vapor or steam which is produced in the press nip and which entrains the water out of the capillaries in the press felt. Also according to the information contained in this document there would be expected damage to the paper web during its exit out of the press nip by virtue of the sudden and uncontrolled expansion of the existing water vapor or steam in the paper web.
In a dewatering apparatus as known from U.S. Pat. No. 4,738,752, granted Apr. 19, 1988, a fibrous web to be dewatered is fed through an extended nip formed between a press member, for example, a press roll and a press shoe. The fibrous web is exposed to pressure . generated by the press shoe with the interposition of blanket means and heat is transferred to the fibrous web, for example, by heating means such as induction heaters which heat the roll surface of the press roll outside of the extended nip. Instead, the press roll may be provided with internal bores for throughpassing a heat carrier or with an externally heated heat transfer band which is passed through the extended nip conjointly with the fibrous web. Furthermore, a water absorbing web like a felt web is passed through the extended nip conjointly with the fibrous web. In a further modification, preheating means are provided for preheating, for instance, the fibrous web prior to its entry into the extended nip.
In this construction, the press shoe has a configuration which permits exposing the fibrous web to different pressure conditions while the fibrous web is passed through the extended nip in contact with the preheated press roll or heat transfer band. During the passage through the extended nip and after first passing through a thermally-augmented wet pressing phase, a large proportion of liquid water which is contained in the fibrous web, is displaced therefrom under the action of water vapor or steam which is generated under the prevailing pressure and temperature conditions. Following the second phase, the fibrous web is subjected to pressurized flash drying during a third phase of the drying operation and to unpressurized flash drying during a fourth phase of the drying operation upon exit of the fibrous web from the pressing section.
One or more pressure shoes can be arranged in juxtaposition at the press roll. If a multiple number of press shoes is employed, the individual shoes are spaced from each other. In the intermediate space, the felt web may be separated from the fibrous web in order to prevent rewetting thereof. Also, additional heating means may be provided in the intermediate space between two press shoes.
With the heretofore employed dewatering methods it has not been possible to obtain the advantages and effects for the dewatering operation which were to be expected by virtue of the simultaneous employment of pressure and temperature in the pressing nip, sometimes simply referred to herein as nip. In particular, it has proven disadvantageous that pressure decreases for flash drying may result in "explosive" evaporation resulting in damage to the web and the temperature is difficult to control and adjust due to the heat transfer along the press nip and because the thermal energy is supplied outside, specifically precedingly or upstream of the press nip as viewed in the travel direction through the press nip.